Best way to check a/f mixture after jet change?
 

I swapped out the primary jets in my 850dp. I went from the stock 80s to a pair of 76s. I put in 4 new plugs (the easiest to get to). How do I need to drive it to determine if the mixture is ok? Easy, WOT, normal cruising? Also, how long do I drive it before checkin the plugs?

Brett :thumbs:

Re: Best way to check a/f mixture after jet change? (Brettmc)
 

I did the same thing here today. I went from 67 to 65 just to see if I could tell a difference. Somebody let us know on this one. I was just gonna drive it and check them on Friday after about 50 miles or so.

Re: Best way to check a/f mixture after jet change? (Brettmc)
 

The a/f mixture would reuire some type of a/f testing system. However if your looking at it from a performance side, you can use a stop watch and do 0-60 test or use one of those G-Tec devices.

Keep testing and increasing jets till increase in performance doesn't increase anymore.

Re: Best way to check a/f mixture after jet change? (Tominator)
 

:iagree:

An a/f gauge or the track is best. You pay attention to trap speed, not time. Keep changing until you see a reversal. If you go extreme toward lean you'll see your tempetature higher too.

Just about the same thing with timing, except you watch your 60 foot speed as well. That will help you dial in your initial and the curve.

Best way to check plugs is immediately after your run, as in right at the side of the road. Checking them in the garage will be checking overall driving/engine conditions (unless, of course, that's what you want)
:cheers:

Re: Best way to check a/f mixture after jet change? (biltogo)
 

I know already after about a 20 mile trip that I don't think it was the right thing for me. I have a slight surging that I didn't have yesterday. So for me the 67 are going back in.

Re: Best way to check a/f mixture after jet change? (Brettmc)
 

I would highly suggest to anyone who thinks they will be making more than a couple diff changes at diff times to invest in a heated O2 sensor. I bought an Edelbrook and it gives ou info you can't get from reading plugs or a stop watch. It is especially good for cruising rpm's, you can lean it out to get way better fuel economy than you can by reading plugs. Matt Gruber is good with it and his website has some info on O2 tuning.

Re: Best way to check a/f mixture after jet change? (Fevre)
 

Thanks guys. Any tips on the kind of a/f gauge to get?

Brett :thumbs:

Re: Best way to check a/f mixture after jet change? (Brettmc)
 

You can use a digital multimeter to check your O2 sensor reading if you have one. An O2 sensor gives an output of .1-.9 volts. Put your meter on mV and your range will be 100-900 mV, with 900 being rich and 100 being lean. Or you can get an air/fuel gauge from Summit or jegs for 30 bucks and use it.

-Justin

Re: Best way to check a/f mixture after jet change? (Dalannex)
 

I have a multimeter. I might just use that. As far O2 sensors, any idea on one to get? I read Gruber's webpage on A/F tuning. If I interpreted it right, it's ok to mount a heated sensor in the collector. Does it matter which header collector it goes into? From what I read, he suggests the driver's side, but is that ok for a big block?

Brett :thumbs:

Re: Best way to check a/f mixture after jet change? (Brettmc)
 

My BB was leanest on cyl 7, so i placed the lambda in the collector on drivers side.

You can build your own A/F meter for about $10, take a look at: http://www.markvanderkwaak.com/dbbp/tech-tips/mm.html

It 's not that easy to tune with an A/F meter either. You have most resolution around 14-15 (chemical ideal) and only a few metering point outside that area. But it's much better than guessing.

Check this diagram:
http://w1.212.telia.com/%7Eu21203717...h_680_ohm2.jpg

The X's are the points that I see on the led's

What's your engine and carb setup now?


[Modified by Lasse, 8:51 AM 7/16/2003]

Re: Best way to check a/f mixture after jet change? (Brettmc)
 

I did a change from 80 to 76 on my 850DP carb, but then I got a stumble from idle. If you get that, try to give the carb some more fuel on the idle screws, maybe 0.5 to 1 turn more than you have now.

Bob2000 has a good site for Holley tuning info:

http://www.bob2000.com/carb.htm

Keep us posted. :cheers:

Re: Best way to check a/f mixture after jet change? (Lasse)
 

Thanks, Lasse. I don't know the cam specs as previous owner had no info on it. It has stock iron heads, Torker II, and of course the new Holley 850dp. The 850 is box-stock except for the 76 primary jets I just put in (replaced the stock 80s). What's the setup on your carb?

That's a real steep curve on the O2 sensor output. Are there any that "flatten" the curve without steppin into wide-band territory?

Brett

Re: Best way to check a/f mixture after jet change? (Brettmc)
 

You can mount your O2 sensor farther away from the exhaust manifold if you run a 4 wire or heated sensor. A non heated sensor needs to be closer to the head in the exhaust because O2 sensors need to be 600 degrees before they will read, so the non heated need to be close to the source of the heat, or the head, to mainitan that heat, especially if you are running no cats. The heated sensors have a built in heater to keep them the proper temp so that they read. They do, of course, need a fused 12 volt supply to them and a ground. I ran a wire down from my fuse box with an in line fuse, then grounded to the frame.

-Justin

Re: Best way to check a/f mixture after jet change? (Dalannex)
 

Excellent info Justin! What's the price range on the heated sensors? Do they all have the same sensitivity?

Brett :thumbs:

Re: Best way to check a/f mixture after jet change? (Brettmc)
 

For idle postion and cruise adj it is fairly easy and accurate once you are in the right territory. The O2 sensor is best when reading at stoic, which is 14.7/1. It gets more tricky once you start tuning for transitions and wot. Hope Matt chimes in, he seems to know his stuff. A wide band one is not really needed for carb tuning, no way you get your carb to tune exactly for every cond but with FI you can get damn close since there are so many other inputs to adj fuel ratios like coolant temp, intake air temp, manifold pressure and also tables for to cross reference many of these. But still a heated O2 sensor is a very usefull tool in my opinion and makes tuning a lot easier. Best to postion it close (12" I think) to your leanest cylinder to get better readings for transitional tuning. But I think Hedman makes a collector reducer with a bung already welded in it so you don't have to put it in your collector and ruin coated headers, and that should work fine for most lay people like me.

Re: Best way to check a/f mixture after jet change? (Fevre)
 

I still do it the old fashion way. I just look at the spark plugs. I was going to buy the $130 A/F gauge and kit. Then I started thinking that once your setup you'ld never use it again.

To determine the correct primary jetting you would need to look at the plugs right after a long freeway cruise. 76 in a 850 cfm would be pretty lean, but it would work on a stock or mildly modified motor. If it's not stumbling when you add a little throttle going down the freeway your OK.

Re: Best way to check a/f mixture after jet change? (Fevre)
 

Fevre - Can ya give me the scoop on the Edelbrock you mentioned?

Brett :thumbs:

Re: Best way to check a/f mixture after jet change? (Brettmc)
 

For some reason Summit doesn't have the sensor avail on it's website so I got this off a comp that i hate and will not mention:

http://www.jegs.com/photos/3506593.jpg

About $130, guess you build one for about half that but I didn't feel like doing it myself. Comes with the meter (shown), a heated sensor and wiring.

Could have sworn I saw a 3" to 2 1/2" Hooker header reducer sold as a single with a prewelded O2 sensor bung for under $30. I'll check at lunch for the part # for both. :thumbs:

Re: Best way to check a/f mixture after jet change? (gkull)
 

Goerge

But if you are changing stuff all the time like me, never happy I guess:), it is very usefull plus it can also give you a way to monitor if you if you are still in wack

Re: Best way to check a/f mixture after jet change? (Fevre)
 

I have the Edelbrock A/F meter and I am frustrated with using it, but maybe I don't know enough about what I am doing! I wanted it for tuning my Qjets on my modified Vette (355 cid) and modified daily driver (305 LG4). Here is my experience. For richening jetting for WOT mixture, I can jet way up and it still doesn't read richer than 13.5 to 1. I can't figure out if this the meter just not reading right, or if increasing jetting (primary side only, secondaries are locked out for tuning primary side first) is only working so much. For example, going from 73 jet to 75 jet increased richness from 14.0 to 1 to 13.5 to 1 at WOT, but moving to 77 jets made no further increase in richness. Anyone understand why? I changed fuel filter and pump thinking the carb wasn't getting enough to supply. Car runs well though and I do see increase richness under cruise if I don't reset the APT stop. The other possibility is the meter is defective for WOT readings. I guess that wouldn't surprise me too much because this is my second unit. The first one stopped working and was replaced by Edelbrock. Both are 3 wire sensors.
The other frustrating thing is the reading I get while cruising depends on the gasoline I am using. I have proven this to myself by switching brands back and forth and it is consistent, but I don't know which value to rely on. On Shell gas, I get 1.0 to 1.5 points richer readings than with Sunoco gas (which contains up to 10% ethyl alcohol)(always 92 octane). Even 10% alcohol shouldn't add enough oxygen to lean the mixture from 14 to 1 to 15 to 1. I suspect it has something to do with burn efficiency or fuel viscosity but thats just guessing. The WOT readings vary much less with the different fuels.
Anyone know what may be going on?
Thanks, Richard